1. Field of the Invention
This invention relates to a novel bioadaptable material capable of dissolving blood clots. More particularly, this invention relates to a bioadaptable material having a fibrinolytic enzyme such as urokinase or tissue plasminogen activator (hereinafter abbreviated as "TPA") immobilized on a bioadaptable porous crystallized glass. This material has the function of continuously converting plasminogen in blood into plasmin.
2. Description of the Prior Art
Blood clots are clots of blood which occur in blood vessels. When blood clots occur, they clog fine peripheral blood vessels in the human body and may cause paralysis of the hands and feet, cerebral infarct, and myocardial infarct.
The main component of blood clots is a scleroprotein called a "fibrin". This fibrin is known be decomposed by the enzyme plasmin. Plasmin is known to be present in the blood generally in the form of plasminogen, a precursor. The thrombolytic agents in popular use today include those which convert this plasminogen into plasmin and those which suppress coagulation of blood [Biochim. Biophys. Acta. Vol. 24 (1957), pp. 278-282 and Agric. Biol. Chem. 55 (5), 1,225-1,232 (1991)].
As substances which convert plasminogen into Plasmin, there can be mentioned fibrinolytic enzymes such as urokinase and TPA. Urokinase specifically acts on plasminogen, severing the alginin-valine bond thereof, and consequently converting the plasminogen into plasmin. In the past, urokinase was obtained by extraction from human urine. In recent years, however mass-production of the urokinase has been realized by the introduction of a method involving the culturing of urokinase-producing renal cells and a method of gene recombination.
TPA exhibits better ability to dissolve blood clots than urokinase because it converts plasminogen into Plasmin through union therewith and, at the same time, unites with fibrin thereby producing an environment allowing the plasmin to act easily with the fibrin. Since TPA occurs only in a very small amount in the uterus and blood vessel walls in human beings, studies have been actively pursued in search of a method for mass-production of TPA by gene recombination or cell culture. Such methods are now on the verge of being commercialized.
When a fibrinolytic enzyme such as urokinase or TPA is used as a thrombolytic agent to combat thrombosis, it is generally administered by direct injection into the patient's blood vessels. In this case, the disadvantage arises that the activity of converting plasminogen into plasmin is quickly deteriorated as by the fibrinolytic enzyme inhibiting factor in the blood and, at the same time, the plasmin activated by the fibrinolytic enzyme is quickly deprived of the thrombolytic activity by the action of the plasmin inhibiting agent in the blood.
Advances in biotechnology in recent years have led to the development of bioreactors whose elements comprise immobilized enzymes (enzymes immobilized on a carrier) and immobilized microorganisms (microorganisms immobilized on a carrier) and such bioreactors have come to be widely used in the production of useful substances, quality control in the process of production of amino acids, the analysis of foodstuffs, and clinical diagnosis.
Various methods have been proposed for the immobilization of enzymes. The carrier bonding method is one. This method accomplishes the deposition of a given enzyme by physically adsorbing the enzyme on a water-insoluble carrier [Biotechnology and Bioengineering, Vol. XXI, pp. 461 to 476 (1979), John Wiley & Sons Inc.], ion bonding and covalent bonding [Science, Vol. 166, pp. 615 to 617 (1969)]. Among these methods, the covalent bonding method is used for the broadest range of enzyme types.
As the water-insoluble carrier, inorganic porous substances are commonly used. When a fibrinolytic enzyme such as urokinase or TPA is bonded to a conventional inorganic porous substance, the porous substance may exude harmful ions through its surface or the fine surface texture of the porous substance may decay and allow the bound fibrinolytic enzyme to escape.
An object of this invention is to provide a bioadaptable material which maintains an enzymatic activity stably and efficiently for a long time and, on being administered into a patient's body of adsorbed on blood vessels, exhibits a thrombolytic activity, i.e. the function of continuously converting the plasminogen in the blood into plasmin.